EDI System Selection: Feed Water Requirements and Modules for RO + EDI
Operation of an EDI module depends on the quality of the RO permeate. Feed water limits, recovery, concentrate flow, DC current, and sanitization: the key parameters for selection and design.
Five Questions When Selecting an EDI Module
- Flow rate: product water flow rate determines the module series (MX 30–500 L/h; LX 0.22–10 m³/h; VNX up to 22.7 m³/h in a single module).
- Feed quality: the RO permeate must meet the FCE, hardness, chlorine, silica, TOC, and metal limits; parameters such as chlorine, iron, and manganese can permanently damage the resin and membranes.
- Target water quality: product water resistivity (>15 MΩ·cm at nominal flow for LX-X; >17 MΩ·cm on RO permeate feed for VNX) and water grade.
- Sanitization: chemical or 85°C hot water (LX-HI)?
- Power and control: DC3, DCR, or G4; pump interlock and low-flow protection.
See our EDI systems and purified water systems pages for system overviews and water grades.
What Are the RO Permeate Limits Entering EDI?
Feed water table from the Ionpure LX manual (Table 2-3). In most cases RO pretreatment satisfies these values, though some facilities require additional pretreatment.
| Parameter | LX-EU, LX-X, LX-Z | LX-HI |
|---|---|---|
| FCE (µS/cm) | ≤ 40 | ≤ 40 |
| Total hardness (CaCO₃) | ≤ 1.0 ppm | ≤ 1.0 ppm |
| Total chlorine (Cl₂) | ≤ 0.02 ppm | ≤ 0.02 ppm |
| Silica (SiO₂) | ≤ 1.0 ppm | ≤ 1.0 ppm |
| Iron, manganese, sulfide | ≤ 0.01 ppm | ≤ 0.01 ppm |
| TOC (as C) | ≤ 0.5 ppm | ≤ 0.5 ppm |
| pH range | 4 – 11 | 4 – 11 |
| Feed water temperature | 5 – 45°C | 5 – 60°C |
| Inlet pressure | ≤ 6.9 bar (100 psig) | ≤ 6.9 bar (100 psig) |
The LX-Z datasheet lists a chlorine tolerance of up to 0.05 ppm in feed water; in system design, using the 0.02 ppm limit from the manual is conservative. In the VNX series, the hardness limit of at most 1 ppm (CaCO₃) is tighter than in other families, with a temperature range of 5–45°C and pH 4–11; it is preferred in critical boiler feedwater applications.
How is FCE calculated?
FCE = measured conductivity (µS/cm) + (ppm CO₂ × 2.79) + (ppm SiO₂ × 2.04). CO₂ and silica are weakly ionized species but represent an ionic load for EDI. Example: for conductivity 6 µS/cm, CO₂ 5 ppm, and SiO₂ 0.5 ppm, FCE = 6 + 13.95 + 1.02 = approximately 21 µS/cm, which is below the LX limit of 40 µS/cm. CO₂ is measured with a test kit, while silica can often be neglected when low. According to the manual, chlorine permanently damages EDI resin, whereas iron, manganese, and organics (TOC) foul resin and membrane surfaces, increasing electrical resistance and pressure drop.
Recovery Rate and Concentrate Stream
Recovery = 100 × product flow / (product flow + concentrate flow). In LX modules, the maximum allowable recovery depends on hardness and silica in the feed water (manual Table 5-2):
| Hardness (ppm CaCO₃) | Silica (ppm SiO₂) | Allowable recovery |
|---|---|---|
| ≤ 0.2 | ≤ 0.5 | 95% |
| 0.2 – 1.0 | 0.5 – 1.0 | 90% |
Concentrate flow is typically approximately 11% of product flow (90% recovery). Example: if LXM45 product flow is 5 m³/h and allowable recovery is 90%, concentrate flow = (100 × 5 / 90) − 5 = 0.56 m³/h. Each module size has a minimum concentrate flow rate that must be maintained; therefore, 95% recovery may not be achievable below nominal flow:
| Module | Minimum concentrate flow rate (L/min) |
|---|---|
| LXM04 | 0.26 |
| LXM10 | 0.61 |
| LXM18 | 1.02 |
| LXM24 | 1.36 |
| LXM30 | 1.67 |
| LXM45 | 2.50 |
The module is installed near a drain capable of accepting the entire total feed flow. Product outlet pressure is maintained approximately 0.1–0.3 bar higher than concentrate outlet pressure; if concentrate leaks into the product side, product water resistivity drops. If concentrate is recycled to the RO inlet, the CO₂ load increases and electrode gases must be vented.
Ionpure Series by Flow Rate and Application
| Series | Flow rate | Product water and operation | Application |
|---|---|---|---|
| MX (MX30–MX500) | 30–500 L/h | 5 bar, up to 45°C; 90–95% recovery; no concentrate pump or brine injection required | Laboratory, small RO systems |
| LX-Z | 0.22–10 m³/h | 90–95% recovery, 90–99% silica removal; 0-400 VDC | Boiler feedwater, HPI/CPI |
| LX-X | 0.22–7.67 m³/h | >17 at minimum flow, >15 at nominal, >7 MΩ·cm at maximum | Power, electronics, food and beverage, laboratory |
| LX-EU | 0.22–7.67 m³/h | Wetted materials compliant with FDA 21 CFR and EU food-contact directives | EU market, food and pharmaceutical water |
| LX-HI | 0.22–7.67 m³/h | 85°C (plus/minus 5°C) hot water sanitization; <0.1 µS/cm; up to 60°C continuous | Pharmaceuticals, biotechnology |
| VNX-Mini / VNX-Max | 12.0 / 15.0 m³/h nominal; up to 22.7 m³/h | >17 MΩ·cm on RO permeate feed; silica and boron >95%; sodium and chloride >99.8% | Boiler feedwater, power, microelectronics |
Feed requirements for special VNX variants
| Variant | Feed FCE | Other feed conditions | Typical product performance |
|---|---|---|---|
| VNX-Ultra | < 10 µS/cm | 20–45°C; inlet pressure 2.1–6.9 bar | ≥18.0 on DI feed, ≥17.5 MΩ·cm on 2-pass RO feed; silica and boron ≥99.8%; sodium and chloride ≥99.9% |
| VNX-HH | < 40 µS/cm | Total hardness up to 2 ppm; 4–45°C | >16 MΩ·cm at nominal flow; recovery 80–90% depending on hardness |
| VNX-CDIT | < 100 µS/cm | Hardness up to 4 ppm, silica up to 2 ppm; 5–45°C | Product water conductivity <0.1 µS/cm; recovery 90–95% |
Variant values are from the Ionpure general catalog; for LX and VNX, the catalog lists product water resistivity at nominal flow as 15–18 MΩ·cm and recovery as 90–95%. According to the Ionpure general catalog, the product family spans from 0.013 m³/h to 22.7 m³/h, and Ionpure first commercialized CEDI in 1987. In the RO + EDI pure water systems we build, Mikron selects Ionpure CEDI modules according to RO permeate quality, target water grade, and flow rate, sizing them together with the power supply.
DC Current, Interlocks, and Start-Up
DC current per module can be roughly estimated by the equation DC amps = 1.31 × (product flow, L/min/cell) × FCE / current efficiency (%) (Faraday's law); the method recommended in the manual is the Ionpure IP-PRO projection tool. In system design, the following requirements from the manual apply:
- The DC power supply is interlocked with the RO or EDI feed pump so that it cannot be energized when the pump is not running.
- Low-flow protection is provided on the product and concentrate lines.
- Prefiltration is installed immediately ahead of the EDI (5 micron recommended); the dead-head pressure of the feed pump does not exceed 6.9 bar.
- When starting from standby, initial RO permeate is diverted to drain because initial permeate is often of poorer quality than RO feed water. Rather than a conductivity target, the manual recommends flushing for a defined duration (3–5 minutes).
- At start-up, product outlet pressure is adjusted 0.1–0.3 bar higher than concentrate outlet pressure.
Power supplies: DC3 (3-phase, 0–600 VDC / 0–15 A, Modbus TCP), DCR (single-phase, 5-step current selection), G4 (electrical resistance logging and cleaning alert). Details: Ionpure power supplies.
When to Clean and How to Sanitize
According to the manual, the module should be cleaned when product or concentrate differential pressure increases by 50% without a change in temperature or flow, when product quality declines without a change in temperature, flow, or feed conductivity, or when module electrical resistance increases by 25% without a change in temperature. These are signs of fouling or scaling. Cleaning chemicals (hydrochloric acid, brine/caustic, sodium percarbonate) and sequence are selected from the manual table according to foulant type, with a brine flush performed between low-pH and high-pH cleanings.
For microbial control, periodic peracetic acid sanitization or — exclusively with LX-HI — 85°C (plus/minus 5°C) hot water sanitization is applied. During hot water sanitization, feed pressure is kept below 2 bar (30 psig), water is recirculated at 85°C (plus/minus 5°C) for 1–2 hours, and heating/cooling does not need to be ramped gradually. Aside from temperature, sanitization water must meet continuous-operation feed water requirements. Afterward, the unit is flushed to drain for 5–10 minutes with DC power off.
When Contacting Us for EDI Selection
- Required product water flow rate and target resistivity / water grade
- RO permeate analysis: conductivity, CO₂, silica, hardness, total chlorine, TOC, iron, manganese, pH, temperature
- RO system flow rate, recovery, and pretreatment (including cartridge filter)
- Sanitization expectation: chemical or hot water; cGMP/USP/Ph.Eur. requirement
- Drain availability for concentrate or whether recycling to the RO inlet is desired
- Electrical supply (3-phase / single-phase) and SCADA communication
Frequently Asked Questions About EDI System Selection
FCE, concentrate stream, prefilter, power supply interlocks, and pharmaceutical applications.
What data are needed to select an EDI system?
Product water flow rate; feed conductivity equivalent (FCE), hardness, total chlorine, silica, iron/manganese/sulfide, TOC, pH, and temperature of the RO permeate; target product water resistivity; sanitization method (chemical or 85°C hot water); drain capacity for reject concentrate; and power supply/control infrastructure. These values are compared against the feed water table in the Ionpure LX manual; any parameter exceeding its limit requires additional pretreatment.
What is FCE (feed conductivity equivalent)?
FCE expresses the ionic load entering the EDI module with a single number. According to the Ionpure LX manual, FCE = measured conductivity (µS/cm) + (ppm CO₂ × 2.79) + (ppm SiO₂ × 2.04). For example, if conductivity is 6 µS/cm, CO₂ is 5 ppm, and silica is 0.5 ppm, FCE is approximately 21 µS/cm; the limit for LX modules is 40 µS/cm. Because CO₂ is a dissolved gas, it is measured with a separate test kit; silica is often low enough to be negligible.
How much is the EDI concentrate (reject) stream, and where does it go?
EDI produces no acid or caustic regeneration waste, but it is not zero-discharge: alongside product water, the module generates a concentrate stream carrying the removed ions. According to the LX manual, this stream is typically approximately 11% of the product water flow (90% recovery). The module is installed near a drain capable of handling the full total feed flow. If the concentrate is recycled to the RO inlet, the CO₂ load increases and may affect product water quality, and venting is required for electrode gases.
Is a 5-micron filter required ahead of EDI?
The Ionpure LX manual states that prefiltration must be installed immediately upstream of the CEDI system and recommends 5 micron, as storage tanks and repumping are common sources of particulate fouling. The dead-head pressure of the feed pump must also not exceed the module's 6.9 bar (100 psig) limit. For cartridge filter selection, see our cartridge filter guide.
How is the EDI power supply controlled?
The DC power supply must be interlocked with the RO or EDI feed pump so that it cannot be energized when the pump is not running. Low-flow protection must also be provided on the product and concentrate lines. DC current per module is calculated from FCE, flow rate, and current efficiency using the Ionpure IP-PRO projection tool or the Faraday equation in the manual. For the DC3, DCR, and G4 power supplies, see our Ionpure page.
Which EDI module is selected in the pharmaceutical industry?
In lines requiring microbial control, LX-HI is selected: it is sanitized with 85°C (plus/minus 5°C) hot water, withstands up to 60°C in continuous operation, delivers product water conductivity below 0.1 µS/cm, has verified performance over more than 150 sanitization cycles, and is constructed with cGMP-, USP-, and Ph.Eur.-compliant materials. For lines requiring food contact compliance in the European Union, LX-EU is available.
Send the make and model of your current instrument to [email protected] — our engineers will propose the equivalent with the same connection and measuring range.
Planning a project? Send us your flow rate, feed water source and target water quality to [email protected] — our engineers will reply with a technical proposal.